This invention pertains to low frequency oscillators and, more particularly, to low frequency oscillators of the astable multivibrator type having good frequency, stability, high gain, fast starting, good immunity to DC loading at the output terminals and which may be fabricated in integrated circuit form.
In the past, various types of oscillator circuits have been used to generate relatively low frequency signals. However, a low frequency necessarily means a correspondingly high time constant. For those types of oscillators employing resistive-capacitive networks, high time constants translate into high resistive values and/or high capacitive values. Of course, high resistive values and high capacitive values necessarily mean that it is impractical to adapt such low frequency oscillators to integrated circuit form. In some instances, the high resistive and/or capacitive values are provided as external components to the integrated circuit. However, this arrangement is not entirely satisfactory since it increases the number of pins or external connections to the integrated circuit and makes the oscillator more susceptible to stray circuit effects such as parasitic capacitance and leakage currents.
Even when low frequency oscillators are designed and built, there is a need in the communications arts, and particularly in private line encoder/decoder applications, for the low frequency oscillator to have a high degree of frequency stability, with respect to temperature and voltage supply variations. As is well-known to the art, private line communication systems typically use a low frequency tone signal to selectively enable communications between selected parties using the same communications channel. The tones used for private line communications are typically below the lowest audio frequencies passed by the communications system. For example, a number of different tones may be employed which have frequencies below about 300 Hz. In order to provide for as many private lines as possible, both the transmitting and receiving radio equipment must have high frequency accuracy and stability so that the tones may be closely spaced without interference from, or falsing due to, adjacent tones.
It is known to the prior art to use an astable multivibrator as an oscillator wherein one of the cross-coupling capacitors in the multivibrator is replaced by a frequency determining element, such as a crystal having predetermined frequency characteristics. However, such an oscillator is limited in its utility since low frequency applications typically require high resistances in the collector circuits of the switching transitors and a relatively high cross-coupling capacitance between the respective transistors in addition to the crystal. As discussed above, such requirements are not generally compatible with integrated circuit fabrication techniques. Furthermore, as the impedance levels increase, the oscillator generally tends to be slower in its starting characteristics. It is important in private line communications applications that the oscillator have a fast starting characteristic such that the desired private line tone can be generated and detected quickly at the desired time in the respective encoders and decoders.
Since the output terminals of the oscillator are intended to provide the oscillator output signal to other circuitry, the output of the oscillator is subject to a load. The load may vary subject to temperature or voltage supply variations or may otherwise be time-varying. It is well known that loading affects start-up performance of oscillators and can further affect frequency stability.